1. Field of the Invention
This invention relates generally to microphone audio signal processing, particularly related to multiplexed microphone signals with multiple signal processing paths.
2. Description of the Related Art
A microphone is a basic and essential element in an audio system. There are many different applications to a variety of audio systems. The most common audio systems include, at least, the following types: a teleconference system, a public addressing (PA) system, a recording studio, or some combination of the above three.
A simplest teleconference system is a telephone. Two people at two physically separate locations may talk to each other through a telephone network and two telephone sets. FIG. 1 illustrates a simplest teleconference system 100. The teleconference system 100 has two sites, a near site and a far site. At each site, there is a telephone, 110 and 150 respectively. The two telephones are connected through a network 130, typically a Public Switched Telephone Network (PSTN), sometime referred to as Plain Old Telephone Service (POTS). The near site telephone 110 has at least a microphone 102 and a loudspeaker 104. Typically, the telephone also has a circuitry or processor module 106 to perform some signal processing. For example, most touch-tone phones can make different tones to represent different number keys, making artificial ring tones that can be changed by a user. The telephone 150 at the far site may or may not have the same components at in the telephone 110. For simplicity, it is assumed that the telephone 150 has at least a microphone 152, a loudspeaker 154 and a processing module 156.
In a more advanced telephone, the processor module 106 may have more circuitry or more processing power to perform many functions. One state of the art telephone is a Polycom SoundStation® VTX-1000 speakerphone, available from the assignee of the current invention. The VTX-1000 has many more features and functions. For example, it is a speakerphone that allows full-duplex mode of operation. In full-duplex mode, talkers at both sites of the conference call can speak at the same time. To allow full-duplex mode of operation, the VTX-1000 has an advanced acoustic echo canceller (AEC). Without an AEC, annoying echo-like sounds will circulate between the two sites. If AEC is not implemented, then the speech signal 172 from a talker at the far site is transmitted through the network 130 to the near site telephone 110 as signal 134. The speech signal 134 is reproduced by the loudspeaker 104. Since the telephone is operating in full-duplex mode, the microphone 102 is active when loudspeaker 104 is working. The microphone 102 generates a signal 132, which contains contributions due to the far end speech signal 172 from the loudspeaker 104. This far end signal embedded in signal 132 is transmitted back to the far end together with the near site speech signal also in signal 132. The entire signal 132 becomes a loudspeaker signal 174 at the far end and reproduced by loudspeaker 154. This way, the far end talker will hear his voice back from the loudspeaker 154, like an echo. This echo speech signal produced by the loudspeaker 154 can again be picked up by microphone 152, transmitted through network 130, reproduced by loudspeaker 104, picked up by microphone 102 and transmitted back to loudspeaker 154. If nothing is done to it, the echo signal can circulate between the two sites for a long time until dissipated into background noise, which is increased due to such echoes. Without AEC, full-duplex mode operation in a speakerphone is not practical due to the echoes and the noise.
When a process module 106 performs echo cancellation, it estimates the contribution of echo in the microphone signal 132 and subtracts that portion from the microphone signal 132. This way, signal 132 only contains signals due to the speech of near site talkers. Therefore, what a far end talker can hear is the speech of near site talkers alone, without echo of his own voice. At the far end, another process module 156 may perform the similar acoustic echo cancellation. To achieve optimal goal of solving the echo problem, besides acoustic echo cancellation, echo suppression and noise fill may also be used. That is to minimize the residual echo heard by participants at the far site.
The process modules 106 and 156 may also perform other audio signal processing. For example, such processing may include parametric equalization. A particular microphone element may not respond to sound with uniform gain for all frequencies. To compensate for this non-uniformity, the process module may apply different filters on different frequencies to enhance or attenuate the frequency to achieve the uniform gain across the spectrum. The process module may also adjust the gain to change the characteristic of the speech or to achieve other acoustic objectives.
The process modules may also include automatic gain control (AGC) to accommodate the different loudness of speech from different talkers. There are various factors that may affect the gain of a microphone to speech, such as the loudness of the talker, the distance between the talker and the microphone or the orientation of the microphone and the talker. The use of AGC can avoid the wide fluctuation of the speech reproduced by a loudspeaker.
Another application of microphone signals is a public addressing system or a sound reinforcement system, as illustrated in FIG. 2. Such a system is typically used in theatres, auditoriums or large classrooms. One of the main differences of system 200 and system 100 is that system 200 is typically used at one site. The microphone 202 and loudspeaker 204 are located at the same general location such that sound from the loudspeaker 204 is picked up by the microphone 202. The microphone 202, process module 206 and loudspeaker 204 can form a closed loop. Unlike system 100, system 200 does not have two sites and cannot have the echo problem. There is no need for acoustic echo cancellation. But it has its own problem, a feedback problem. If the closed loop has an overall gain above unity for a particular frequency, then for that frequency, system 200 has a positive feedback loop which reinforces itself until it makes a very loud squeaky noise, typically referred to as howling. The howling is very disruptive to meetings, lectures or artistic performances. It may also be destructive to acoustic equipment involved in the loop. Eliminating or avoiding feedback is a major concern in making and operating an audio reinforcement system 200. In doing so, a slight degradation of the acoustic performance is acceptable. A typical method for eliminating feedback is to reduce the overall gain below unity for all frequencies. This may limit the amount of amplification in the reinforcement system, which is the main purpose of using such a system in the first place. More advanced methods to avoid feedback can dynamically detect and attenuate only the frequency that is likely to cause the howling, while keeping the gain for other frequencies intact, i.e., the gain for other frequencies possibly can be above unity. The selective attenuation of some frequencies can affect the sound quality, due to the missing portion of the spectrum and the artificial distortion.
As illustrated in FIG. 2, process module 206 may also perform many microphone signal processes 212, including parametric equalization (PEQ), noise cancellation (NC), feedback elimination (FBE), dynamic process compression (DP), automatic gain control (AGC), and automatic mixing (AM). After performing desired processes on the microphone signal, the signal may be amplified by an amplifier 214 to form a loudspeaker signal 234. Loudspeaker signal 234 is reproduced by a loudspeaker 204.
FIG. 3 illustrates another system 300, typically used in sound recording studios, radio broadcasting stations or court recorders. System 300 has a microphone 302, a process module 306 and a recorder or other equipment 304. The main difference between system 300 and systems 100 and 200 discussed earlier is that there is no closed loop in system 300. The microphone 302 generates a signal 332, processed by process module 306, sent to recorder 304 (or other equipment for signal disposal) and that is the end of the system. There is no feedback from the processed signal to microphone 302. Therefore, there is no need to perform some of the processes discussed in systems 100 and 200, namely the echo cancellation, echo suppression and feedback elimination. Without the limitations imposed by the AEC and FBE processes, system 300 is typically focused on achieving the best sound quality possible, which is a requirement in a typical sound recording studio for recording a music performance or for a radio broadcasting stations for transmitting a live performance. When such a system is used for a court recorder, reliability is paramount, i.e., all words spoken or sounds must be recorded. In a typical system 300, the microphone signal processes 312 may include PEQ, NC, DP and AGC etc.